In genomes of species such as the human it is estimated that on average 1 in 103 nucleotides is variant between any two equivalent chromosomes. Although most such variations will be functionally neutral, a small proportion will underlie human phenotypic differences including the risk of disease. DNA variations may be investigated by determining the extent of hybridization of allele specific probes against DNA segments containing the locus of the variation. In this way, it is possible to record ‘matches’ (presence of DNA identical to the probe) and ‘mis-matches’ (presence of DNA non-identical to the probe) for DNA samples from individuals under investigation. However, a problem with existing methods of this type is that it is difficult to determine an adequately discriminatory hybridization stringency. An improved method of increased reliability and simplicity is therefore much needed.